1. Field of the Invention
The present invention relates to a spectroscope for measuring the wavelength distribution (wavelength spectrum of light) of light to be measured (light for measurement) such as light reflected by a sample or light transmitted through a sample to conduct an analysis of the sample.
2. Description of the Background Art
In a conventional spectroscope, light for measurement is transmitted through a slit, changed into collimated light (parallel rays) by a collimator such as a mirror or a lens for collimation, and then incident on a rotatably supported diffraction grating. The collimated light is diffracted by the diffraction grating. The light diffracted by the diffraction grating has a diffraction angle depending on the wavelength thereof Thus, when a diffracted-light focusing member such as a lens or a mirror is disposed in the path of diffracted light having a certain wavelength, the diffracted light with this wavelength is focused to a predetermined focus position. However, the diffracted light with a different wavelength is not focused to the same focus position when the diffraction grating remains motionless. On the other hand, when the diffraction grating is rotated, diffracted light with a different wavelength is sequentially focused to the same focus position.
Since the wavelength of diffracted light focused to the focus position is determined by the rotational angle of the diffraction grating, the detection of the light amount distribution based on the wavelength of the diffracted light (wavelength spectrum of light for measurement) is carried out by placing a slit and a light amount detector for detecting the amount of focused light at the focusing point, and detecting the amount of received light by the light amount detector while the diffraction grating is rotated.
When the light amount detector disposed at the focus position has a very small light-receiving area, the slit at the focus position may be omitted.
Since the diffraction grating has diffraction efficiency depending on wavelength, it is necessary to use a plurality of diffraction gratings with different grating constants and different blazes in order to make measurements over a wide range of wavelengths. The diffraction grating diffracts light rays with wavelengths of different orders at the same diffraction angle. Thus, the light rays with wavelengths of different orders needs to be separated by using an appropriate filter.
The light amount detector disposed at the focus position of the diffracted light has varying sensitivity to wavelengths of incident light or has no sensitivity to some wavelengths. A light amount detector having excellent sensitivity to light with a real wavelength may not be able to sensitively detect light with different wavelength far from the wavelength at maximum sensitivity.
To detect the wavelength spectrum of the light for measurement over a wide range of wavelengths with high sensitivity, it is necessary to dispose a plurality of light amount detectors having different wavelength sensitivity at a plurality of focus positions of diffracted light rays with separate wavelengths such that the light rays with different wavelengths can be sensitively detected.
The wavelength spectrum of the light can be detected sensitively over a wide range of wavelengths by using the plurality of focus positions for the light diffracted by the diffraction grating and disposing the light amount detectors with different wavelength sensitivity characteristics at the plurality of focus positions, respectively.
To use the plurality of focus positions for the light diffracted by the diffraction grating, however, it is necessary to dispose diffracted-light focusing members such as lenses or mirrors in the paths of the diffracted light with different wavelengths sent from the diffraction grating so as to focus the diffracted light with the plurality of wavelengths at the plurality of focus positions, respectively. The arrangement of filters and light amount detectors at the plurality of focus positions allows the detection of the wavelength spectrum of the light over the wide range of wavelengths with excellent sensitivity. However, in that case, the plurality of diffracted-light focusing members are required, and the size of whole apparatus may be increased or higher cost is needed.
According to a different type of conventional method, a wide wavelength spectrum of light is measured as follows.
For example, three diffracting gratings with different grating constants consisting of a visual light diffraction grating, an infrared diffraction grating and an ultra-violet diffraction grating are attached to three sides of a triangular prism rotatable around an axis, respectively. The triangular prism is rotated so that the respective diffraction gratings attached to the sides thereof are positioned sequentially to measurement positions (incident positions of collimated light for measurement). Then, while the diffracting gratings positioned at the measurement positions are rotated, diffracted light is focused on a focus position by a diffracted-light focusing member disposed in the optical path of the diffracted light sent from the diffraction grating. A filter or a light amount detector disposed at the focus position is replaced according to the type of the three diffraction gratings. The filter or the light amount detector appropriate for the employed diffraction grating is used to measure the wavelength spectrum of the focused diffracted light.
In this method of measuring the wavelength spectrum, the three diffraction gratings with different grating constants need to be rotated sequentially to the measurement positions (the incident positions of the collimated light for measurement) for making measurements. In addition, since the filter or the light amount detector disposed at the focus position must be replaced in accordance with a wavelength range to be measured, it takes a long time to measure the wavelength spectrum of the light for measurement.